Compression train including two centrifugal compressors and lng plant including two centrifugal compressors

ABSTRACT

The compression train includes an engine, a first centrifugal compressor driven by the engine and a second centrifugal compressor driven by the engine; the first centrifugal compressor is housed inside one case; the second centrifugal compressor is housed inside one case; the first centrifugal compressor has a first inlet fluidly connected to a line of high molecular weight gas, in particular higher than 40; the second centrifugal compressor has a second inlet fluidly connected to a line of low molecular weight gas, in particular between 20 and 30; the second centrifugal compressor is arranged to provide a compression ratio higher than 10:1.

TECHNICAL FIELD

Embodiments of the subject matter disclosed herein correspond tocompression trains including two centrifugal compressors and LNG[=Liquefied Natural Gas] plants including two centrifugal compressors.

BACKGROUND OF THE INVENTION

FIG. 1 shows a schematic diagram of an LNG plant 100 according to theprior art, in particular a plant implementing an APCI process, i.e. awell-known liquefaction technology with a first cycle using onepure-refrigerant and a second cycle using one mixed-refrigerant.

The plant 100 consists of a first compression train with a centrifugalcompressor 130 and a centrifugal compressor 160, having a first commonshaft, and a second compression train with a centrifugal compressor 140and a centrifugal compressor 150, having a second common shaft. Thecompressor 130 is used for compressing propane; an inlet 131 ofcompressor 130 is fluidly connected to a line of propane; an outlet 132of compressor 130 provides compressed propane. The compressors 140, 150and 160 are used for compressing a mixed-refrigerant gas; an inlet 141of compressor 140 is fluidly connected to a line of mixed refrigerant;an outlet 142 of compressor 140 is fluidly connected to an inlet 151 ofcompressor 150; an outlet 152 of compressor 150 is fluidly connected toan inlet 161 of compressor 160; an outlet 162 of compressor 160 providescompressed mixed refrigerant.

The first compression train is driven by a first engine 110, and thesecond compression train is driven by a second engine 120. The firstengine 110 and the second engine 120 are low speed engines and may befor example an electric engine rotating at a speed of e.g. 1500 RPM or agas turbine rotating at a speed of e.g. 3000 or 3600 RPM.

Each of the compressors 130, 140, 150 and 160 is housed inside adistinct case.

An LNG plant is known from WO 2008/015224 wherein there is a firstcompression arrangement for propane and a second compression arrangementfor a so-called “mixed refrigerant” (i.e. a mixture of hydrocarbonshaving different molecular weights). According to the example process ofFIG. 2, the mixed refrigerant is subject to a compression of 18.5. Atthe priority date of WO 2008/015224, compression of a mixed refrigerantwas typically carried out through three compressors inside threedistinct cases; this also applies to the solution of WO 2008/015224 thatreflects the solution shown in FIGS. 2 and 3 of the article by Perezentitled “The 4.5 MMTBA LNG Train—A Cost Effective Design” (cited by WO2008/015224); therefore, it is to be noted that block 122 in FIGS. 1 and2 of WO 2008/015224 corresponds to three compressors in three cases.Furthermore, according to WO 2008/015224 the first compressionarrangement and the second compression arrangement rotate at the samespeed (i.e. there is no gearbox provided), while the power ratio ofthese compression arrangements can be freely chosen.

SUMMARY OF THE INVENTION

It would be desirable to provide an LNG plant with a reduced number ofcompressor cases with respect to the prior art solutions; this is alsoadvantageous from the footprint point of view.

In general, it is advantageous to increase efficiency, availability andmodularity of LNG plants and to reduce CAPEX for LNG plants.

The above-mentioned objects and advantages apply in particular to LNGplants implementing an APCI process.

Some embodiments of the subject matter disclosed herein relate tocompression trains.

According to such embodiments, the compression train comprises anengine, a first centrifugal compressor driven by the engine and a secondcentrifugal compressor driven by the engine; the first centrifugalcompressor is housed inside one case; the second centrifugal compressoris housed inside one case; the first centrifugal compressor has a firstinlet fluidly connected to a line of high molecular weight gas, inparticular higher than 40; the second centrifugal compressor has asecond inlet fluidly connected to a line of low molecular weight gas, inparticular between 20 and 30; the second centrifugal compressor isarranged to provide a compression ratio higher than 10:1, in anembodiment, higher than 15:1.

Additional embodiments of the subject matter disclosed herein relate toLNG plants.

According to such embodiments, the LNG plant comprises a compressiontrain; the compression train comprises an engine, a first centrifugalcompressor driven by the engine and a second centrifugal compressordriven by the engine; the first centrifugal compressor is housed insideone case; the second centrifugal compressor is housed inside one case;the first centrifugal compressor has a first inlet fluidly connected toa line of high molecular weight gas, in particular higher than 40; thesecond centrifugal compressor has a second inlet fluidly connected to aline of low molecular weight gas, in particular between 20 and 30; thesecond centrifugal compressor is arranged to provide a compression ratiohigher than 10:1, in an embodiment, higher than 15:1.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and constitutean integral part of the present specification, illustrate exemplaryembodiments of the present invention and, together with the detaileddescription, explain these embodiments. In the drawings:

FIG. 1 shows a schematic diagram of an LNG plant according to the priorart;

FIG. 2 shows a schematic diagram of embodiments of a compression train;

FIG. 3 shows a schematic diagram of an embodiment of a compressor thatmay be a component of the compression train of FIG. 2; and

FIG. 4 shows a schematic diagram of an embodiment of a LNG plant.

DETAILED DESCRIPTION

The following description of exemplary embodiments refers to theaccompanying drawings.

The following description does not limit embodiments of the invention.Instead, the scope of embodiments of the invention is defined by theappended claims.

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with an embodiment is included inat least one embodiment of the subject matter disclosed. Thus, theappearance of the phrases “in one embodiment” or “in an embodiment” invarious places throughout the specification is not necessarily referringto the same embodiment. Further, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

In the following (and according to its mathematical meaning) the term“set” means a group of one or more items.

The compression train 200 of FIG. 2 comprises an engine 210, a firstcentrifugal (i.e. centrifugal flow) compressor 220 driven by the engine210 and a second centrifugal (i.e. centrifugal flow) compressor 230driven by the engine 210. The first centrifugal compressor 220 is housedinside one case; the second centrifugal compressor 230 is housed insideone case. The first centrifugal compressor 220 has a first inlet fluidlyconnected to a line of high molecular weight gas, in particular higherthan 40; the second centrifugal compressor 230 has a second inletfluidly connected to a line of low molecular weight gas, in particularbetween 20 and 30. Therefore, the gas processed by the compressor 220and then provided at a first outlet 222 is different from the gasprocessed by the compressor 230 and then provided at a second outlet232.

The second centrifugal compressor 230 is a high-compression-ratiocompressor; in particular, it is arranged to provide a compression ratiohigher than 10:1, in an embodiment, higher than 15:1.

A train identical or similar to the one shown in FIG. 2 is arranged toprovide both compressed propane and compressed mixed refrigerant forimplementing an APCI process. In this case,

the high molecular weight gas mentioned above is propane, and

the low molecular weight gas mentioned above is a mixed-refrigerant gas,in particular mixture of propane, ethylene or ethane, and methane.

The train of FIG. 2 comprises only two centrifugal compressors.

FIG. 2 shows two sets of embodiments. According a first set, there isone shaft and the second compressor 230 is directly mechanicallyconnected to the first compressor 220. According a second set, there twoshafts and the second compressor 230 is indirectly mechanicallyconnected to the first compressor 220 through a gear box 250. In FIG. 2,the gear box is drawn with dashed lines as it is optional.

The following applies to the first set of embodiments.

The compression train has a single shaft.

The engine 210 may be an electric motor or a steam turbine or a gasturbine, in particular an aeroderivative gas turbine.

The engine 210 is a high speed engine having, in an embodiment, amaximum rotation speed in the range of 5000-9000 RPM, more particularlya maximum rotation speed in the range of 6000-9000 RPM.

The following applies to the second set of embodiments.

The compression train has two shafts.

The second centrifugal compressor 230 is mechanically connected to thefirst centrifugal compressor 220 through a gear box 250 having atransmission ratio, in an embodiment, higher than 2:1.

The engine 210 is an electric motor or a steam turbine or a gas turbine,in particular an aeroderivative gas turbine.

The engine 210 is a low speed engine having, in an embodiment, a maximumrotation speed in the range of 1500-5000 RPM, more particularly amaximum rotation speed in the range of 1500-4000 RPM.

The following applies to both sets of embodiments.

The train may comprise further an auxiliary engine, in an embodiment,electric motor, such as the engine 240 in FIG. 2. In FIG. 2, the engine240 is directly connected, for example, to the second compressor 230.

It is to be noted that the auxiliary engine may be used at start-up ofthe train and/or to help the main engine when the power absorbed by thecompressor or compressors exceeds certain thresholds; such auxiliaryengine is sometimes called “helper”.

According to the embodiment of FIG. 3, the high-compression-ratiocompressor 230 is a high-compression-ratio centrifugal (i.e. centrifugalflow) compressor and comprises a first set of impellers (i.e. one ormore impellers) and a second set of impellers (i.e. one or moreimpellers) arranged downstream or upstream (in an embodiment,downstream) the first set of impellers.

As shown in FIG. 3, the first set includes two impellers 311 and 312,but any number of impellers from 1 to e.g. 20 is suitable. According tothis embodiment, the second set includes three impellers 321 and 322 and323, but any number of impellers from 1 to e.g. 20 is suitable. Theimpellers 311 and 312 of the first set are centrifugal and unshrouded.As shown in FIG. 3, the impellers 321 and 322 and 323 of the second setare centrifugal and shrouded. At least impellers 311 and 312 and 321 and322 and 323 of the first set and of the second set are housed inside onecase 300. The impellers 311 and 312 and 321 and 322 and 323 of the firstset and of the second set are coupled to each other through mechanicalconnections.

According to an alternative embodiment, all the impellers arecentrifugal and shrouded.

The sets of axial compression stages may be more than two, for examplethree or four.

There may be one or more auxiliary inlets.

There may be one or more auxiliary outlets.

In an embodiment, as in the embodiment of FIG. 3, at least some of theimpellers of said high-compression-ratio centrifugal compressor arestacked on each other and mechanically coupled by means Hirth joint. Thestacked and coupled impellers are tightened together by means of a tierod, in this way, a very stable and reliable mechanical connection isachieved. Each impeller has for example a passing hole at its rotationalaxis and is configured so that the tie rod can pass through it. A rotoris achieved when the impellers are stacked and tightened together.

In the embodiment of FIG. 3 all impellers 311, 312, 321, 322, 323 of thetwo sets are stacked, coupled by Hirth joints 340A, 340B, 340C, 340D,and tightened together by a tie rod 330.

Compressor 230 has a main inlet 301 (labelled 231 in FIG. 2), a mainoutlet 302 (labelled 232 in FIG. 2), and at least one auxiliary inletand/or at least one auxiliary outlet at an intermediate position alongthe flow path from the main inlet 301 to the main outlet 302; FIG. 3shows the general case of one intermediate tap 303, being in someembodiments an auxiliary inlet (see upward arrow) and being in someembodiments an auxiliary outlet (see downward arrow).

In an embodiment, as in the embodiment of FIG. 3, the second set ofimpellers (321 and 322 and 323) are downstream the first set ofimpellers (311 and 312), and the impellers (321 and 322 and 323) of thesecond set may have a smaller diameter than the impellers (311 and 312)of the first set.

According to the embodiment of FIG. 3, the impellers of the first set ofimpellers (311 and 312) are unshrouded and with a larger diameter thanthose of the second set of impellers (321 and 322 and 323).

Unshrouded impellers can rotate faster than shrouded impellers, due tothe absence of the shroud; in fact, when the impeller rotates the shroudis pull outwardly by the centrifugal force acting on it and over acertain rotary speed the shroud risks to pull out the impeller.

Thanks to the rotor configuration of the high-compression-ratiocentrifugal compressor defined above, the compressor can rotate fasterthan traditional centrifugal compressors thus achieving a greatercompression ratio.

It is to be noted that unshrouded impellers and shrouded impellers mayalternate between each other; this happens, in particular, when there isone or more auxiliary inlets and/or outlets.

Centrifugal compressors identical or similar to the one shown in FIG. 3may rotate very quickly and so they can reach a very high compressionratio. Therefore, a single innovative centrifugal compressor in a single(and small) case may replace two or three or more traditionalcentrifugal compressors in distinct cases.

Furthermore, thanks to high rotation speeds of the impellers, high flowcoefficients may be obtained.

By using a train identical or similar to the one shown in FIG. 2 (inparticular with a compressor identical or similar to the one shown inFIG. 3), a high LNG production may be obtained in a smaller space and/orin a smaller footprint and with a lesser number of machines.

It is to be noted that having only one case instead of two or more casesis advantageous from many points of view:

it simplifies installation and maintenance,

it reduces maintenance time,

it increases reliability (less components and less likelihood offailure),

it reduces footprint and weight of machines,

it reduces leakages of gasses,

it reduces the complexity and size of the lubricant oil system.

A train identical or similar to the one shown in FIG. 2 is mainlydesigned to be used in a LNG plant.

FIG. 4 shows a schematic diagram of an embodiment of a LNG plantcomprising two such trains; gear boxes are not shown but may be present.

In such embodiment, both trains are identical.

In such embodiment, both trains implement an APCI process.

In such embodiment, both trains comprises a compressor identical orsimilar to the one shown in FIG. 3.

A plant such as the one shown in FIG. 4 may have a power substantiallyequal to the plant of FIG. 1. Anyway, one of the advantages of the plantof FIG. 4 with respect to the plant of FIG. 1 is that if one componentof the plant breaks the plant of FIG. 1 is not able to produce any LNGwhile the plant of FIG. 4 will be able to produce 50% of the ratedproduction.

This written description uses examples to disclose the invention,including the preferred embodiments, and also to enable any personskilled in the art to practice the invention, including making and usingany devices or systems and performing any incorporated methods. Thepatentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those skilled in the art. Suchother examples are intended to be within the scope of the claims if theyhave structural elements that do not differ from the literal language ofthe claims, or if they include equivalent structural elements withinsubstantial differences from the literal languages of the claims.

What is claimed is:
 1. A compression train comprising an engine, a firstcentrifugal compressor driven by the engine and a second centrifugalcompressor driven by the engine; wherein the first centrifugalcompressor is housed inside one case; wherein the second centrifugalcompressor is housed inside one case; wherein the first centrifugalcompressor has a first inlet fluidly connected to a line of highmolecular weight gas; wherein the second centrifugal compressor has asecond inlet fluidly connected to a line of low molecular weight gas;and wherein the second centrifugal compressor is arranged to provide acompression ratio higher than 10:1.
 2. The compression train of claim 1,wherein the high molecular weight gas is propane, wherein the lowmolecular weight gas is a mixed-refrigerant gas.
 3. The compressiontrain of claim 1, wherein the engine is an electric motor or a steamturbine or a gas turbine.
 4. The compression train of claim 3, whereinthe engine is a high speed engine.
 5. The compression train of claim 1,wherein the second centrifugal compressor is mechanically connected tothe first centrifugal compressor through a gear box having atransmission ratio higher than 2:1.
 6. The compression train of claim 5,wherein the engine is an electric motor or a steam turbine or a gasturbine, in particular an aeroderivative gas turbine.
 7. The compressiontrain of claim 5, wherein the engine is a low speed engine.
 8. Thecompression train of claim 1, comprising further an auxiliary engine. 9.The compression train of claim 1, wherein the second centrifugalcompressor comprises a first set of impellers and a second set ofimpellers; the impellers of the first set being centrifugal andunshrouded; and the impellers of the second set being centrifugal andshrouded.
 10. The compression train of claim 1, wherein the secondcentrifugal compressor comprises a first set of impellers and a secondset of impellers; the impellers of the first set being centrifugal andshrouded; and the impellers of the second set being centrifugal andshrouded.
 11. An LNG plant comprising a compression train according toclaim
 1. 12. The LNG plant of claim 10, further comprising twocompression trains.
 13. The LNG plant of claim 11, wherein the or eachfirst centrifugal compressor is arranged to compress a high molecularweight gas, wherein the or each second centrifugal compressor isarranged to compress a low molecular weight gas; the or each firstcentrifugal compressor and the or each second centrifugal compressorcooperating to liquefy a flow of natural gas.
 14. The compression trainof claim 1, wherein the high molecular weight gas is higher than
 40. 15.The compression train of claim 1, wherein the low molecular weight gasis between 20 and
 30. 16. The compression train of claim 1, wherein thecompression ratio is higher than 15:1.
 17. The compression train ofclaim 2, wherein the mixed-refrigerant gas is a mixture of propane,ethylene or ethane, and methane.
 18. The compression train of claim 3,wherein the engine is an aeroderivative gas turbine